Aircraft weight and center of gravity computer



May 19, 1970 J. A. ELFENBEIN ET AL AIRCRAFT WEIGHT AND CENTER OF GRAVITYCOMPUTER Filed Aug. 23, 1967 FlG.-l

5 Sheets-Sheet 1 FIG.2

INVENTORS JACK A. ELFENBEIN MANFRED CARL MUELLER ATTORNEY May 19, 1970J. A. ELFENBEIN ETAL 3,513,300

AIRCRAFT WEIGHT AND CENTER OF GRAVITY COMPUTER Filed Aug. 25, 1967 5Sheets-.-Sheet 73 FIG.-4

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II I h INVENTORS JACK A. ELFENBEIN o CD (a BY MANFRED CARL MUELLER m N NI Mffihw A TTORNE Y J. A. ELFENBEIN EI'AL 3,513,300

May 19, 1970 AIRCRAFT WEIGHT AND CENTER OF GRAVITY COMPUTER Filed Aug.23, 1967 5 Sheets-Sheet 8 m wi s 5% 2 :2; 22%: m mu 2 2 mnw M m 2+ Am filllllllll II] J flm Mm m 5 M 1 M Q M h o; m m H F i 1 L Z I] Illlll 2: h52 g 32:;

J g mm A TTO RNEY May 19, 1970 J. A. ELFENBEIN ETA!- 3,513,300

AIRCRAFT WEIGHT AND CENTER OF GRAVITY COMPUTER Filed Aug. 23, 1967 5Sheets-Sheet 4 R9 M G f INVENTORS OK A ELFENBEIN JA 82% MANFRED WATTORNEY INVENTORS JACK A. ELFENBEIN BY 1221:3 36. MULLEFf ATTORNEY J AELFENBEIN ETAL 3,513,300 AIRCRAFT WEIGHT AND CENTER OF GRAVITY COMPUTER5 SheetsSheet 5 May 19, 1970 Flled Aug, 23, 1967 United States Patent3,513,300 AIRCRAFT WEIGHT AND CENTER OF GRAVITY COMPUTER Jack AsherElfenbein, 260 Summit Ridge Drive, Beverly Hills, Calif. 90210, andManfred Carl Mueller, Los Angeles, Calif.; said Mueller assignor to saidElfenbein.

Filed Aug. 23, 1967, Ser. No. 662,737 Int. Cl. G06f 7/12; G01m N12 US.Cl. 235-150.2 10 Claims ABSTRACT 0F THE DISCLOSURE This invention ischaracterized by the use of strain gauges mounted in the struts of thelanding gear of an airplane. The strain gauges are connected to acomputing circuit which can solve the problem This invention relates toa computing circuit and more Particularly to a computing circuit forproviding a direct indication of the center of gravity of an airplane interms of percent MAC.

The importance of loading an airplane so its center of gravity fallsbetween predetermined limits along the mean aerodynamic chord (MAC) iswell known. Suffice it to say that if the airplane is loaded so thelocation of the center of gravity falls outside these limits, theairplane will not fly at all, or will not fly safely.

It is also desirable that a comprising circuit for making thesemeasurements be self-contained in the airplane in the event the airplanemust be reloaded on airfields which do not have other means forproviding this information. What is needed, therefore, and comprises animportant object of this invention, is to provide a self-containeddirect reading apparatus for precisely loeating the position of thecenter of gravity of the airplane in terms of percent MAC.

This and other objects of this invention will become more apparent whenbetter understood in the light of the specifications and accompanyingdrawings, wherein:

FIG. 1 is a side view of an airplane in which the selfcontained computeris mounted, showing diagrammatically the location of the meanaerodynamic chord and the center of gravity of the airplane, in relationto the landing gear of the airplane.

FIG. 2 is a side sectional view of a strut showing a pressure transducermounted in the strut cylinder.

FIG. 3 shows the weight measuring circuit portion of the computercircuit.

FIG. 4 shows the weight variable voltage divider used in the weightmeasuring portion of the computer circuit.

FIG. 5 shows the portion of the computer circuit which measures thecenter of gravity in terms of percent MAC.

FIG. 6 shows the percent MAC variable voltage divider used with theportion of the circuit for directly measuring the percent MAC.

FIG. 7 shows a complete computer circuit with the switching element usedto produce the circuit shown in FIG. 3 and FIG. 5.

Patented May 19, 1970 FIG. 8 shows the six positions of the 9 stack 6position switch.

FIG. 9 shows a side elevational view of the 9 stack 6 position switch.

Referring now to FIG. 1 of the drawing, an airplane indicated generallyby the reference numeral 10 is in this particular embodiment, providedwith a tricycle landing gear. The tricycle landing gear includes a noselanding gear 12 and port and starboard landing gears 14 and 16. It is,however, to be understood that the principles of this invention areequally applicable to airplanes with a rear landing wheel or skidinstead of a nose landing gear.

As shown in FIG. 1 the separation between the nose landing gear and theport and starboard landing gear is a distance L. The total weight of theairplane is indicated by the symbol W The center of gravity is adistance Z measured from a reference point, which in this case is chosenas the axis of the port and starboard wheels. The mean aerodynamic chord17, hereafter referred to as MAC, is shown as a straight line 17 on thelongitudinal axis of the airplane and has a length designated as MAC.The distance between the nose landing gear and the leading edge of theMAC is designated as I, and the distance from the leading edge of theMAC and the center of gravity is designated as y. The limits along theMAC in which the center of gravity must fall in any loadingconfigurations l and I W is the weight carried by the nose landing gearand includes the weight on the nose W as well as its unsprung weight WFor equilibrium conditions taking the moments around the reference pointnt t Rearranging Equation 1;

Z W L Wt By definition the percent The diagram in FIG. 1 shows:

+y+z hence AZL ET.LT? WPM Z. L MAC MAC MAC MAC MAC (4) Rearran gin gEquation 4;

m r al; L) L 2 MAC MAC 1. MAC 1. (5)

and from (2) in: in (1 4 L rm MAC MAC L MAC Wt (6) multiplying throughby W L L M %b MZ( i)m 7) and transposing 13%7ZZY? *i) H E (WM) :0

Equation 8 permits the percent MAC to be determined if the total weightof the airplane W, and the weight carried by the nose landing gear plusits unsprung weight W is known. This is because regardless of theloading configuration of a particular airplane, L, l and MAC are knownconstants.

To solve Equation 8 in a self-contained apparatus mounted in anairplane, each landing gear contains a strut cylinder 20, a piston 22,and a wheel 24. As shown by way of example in FIGS. 2 and 3, pressuretransducers 26, 28 and 30 are mounted in the nose and the port andstarboard strut cylinders respectively. These transducers may be of thestrain gage bridge variety and they produce a voltage output which isproportional to the pressure in the strut cylinder. This pressure isitself proportional to the weight carried by the strut cylinder so thatthe voltage output of these transducers is proportional to the weightcarried by the respective landing gears. In addition, it is apparentthat the sum of the voltage outputs from these transducers isproportional to the total weight of the airplane which is one of thefactors needed to solve Equation 8. In addition, the voltage output ofthe nose transducer 26 is proportional to the weight W carried by thefront cylinder, and is another factor required by Equation 8. When thesefactors are inserted in Equation 8 along with the values for l, L andMAC of the airplane, it is apparent that the percent MAC can becomputed. A self-contained direct measurement of the weight isimportant, however, because it is important to know the ex act weight ofthe airplane before take off to make certain the airplane is notoverloaded at take off and to get a better estimate of the fuelconsumption.

The weighing circuit shown in FIG. 3, indicated generally by thereference numeral 32, is a portion of the complete computer circuitdescribed below. This circuit takes the voltage outputs from the nosetransducer 26 and the port and starboard transducers 28 and 30, andfeeds them into a first summing amplifier 34. The negative voltageoutput from the summing amplifier 34 at point W in the circuit is thesummation of the voltage output from all the transducers plus anadditional voltage factor from resistors R and R, which correspond tothe total unsprung weight on the landing gears. Consequently, the outputof the summing amplifier 34 is a voltage proportional to the totalweight carried by the airplane.

An incremental voltage divider 36 is provided, see FIG. 3 and FIG. 4.The input to the incremental voltage divider is selected so its outputis opposite in phase to the output of summing amplifier 34. In theembodiment shown a volt positive input is selected. As seen, the voltagedivider 36 comprises four branches or sets of resistances in parallel toeach other. The maximum take off weight is also known because that is afixed characteristic of the airplane. Consequently, the particularvoltage at terminal W corresponding to the maximum take off weight isalso known. It is clear that the input voltage to the incrementalvoltage divider 36 can be no less than this particular voltage and thisis one of the limiting factors in selecting the input voltage to theincremental voltage divider. The fifteen volt input described above issomewhat greater than the maximum take ofi voltage that would everappear at point W in the circuit.

The output of voltage divider 36 must be added to the voltage output ofsumming amplifier 34 at the input to summing amplifier 38. Since theoutput from the voltage divider is opposite in phase to the output ofamplifier 34, when the output voltage from the summing amplifier 36 isequal in magnitude to the output of the voltage divider 36, the outputof the summing amplifier 38 would be zero and this would be indicated onthe null meter 40.

Consequently, the voltage divider 36 must be both adjustable andcalibrated because when the setting on the voltage divider 36 produces anull indication on meter 40 the total weight of the airplane must bereadable on the calibrations on the voltage divider.

To make these readings easy and accurate, the voltage divider 36 isdivided into four branches designated as A, B, C and D respectively. Inthe particular embodiment shown, each branch comprises a plurality ofresistances totaling 20,000 ohms each, which are connected between +15volts and ground. In addition, each branch has a correspondinglylettered slider movable between the taps of the series connectedresistances. Each tap in Branch D is designed to measure increments of100 pounds of Weight. Each tap in Branch C is designed to measureincrements of 1,000 pounds. Each tap in Branch B is designed to indicateincrements of 10,000 pounds, and each tap in Branch A indicatesincrements of 100,000 pounds of weight. Since the total permissiblemaximum weight of the airplane is known from airplane designconsiderations, the resistances R R R and R and the resistances betweenthe taps in the respective branches may be determined using a simpleapplication of Ohms law. These resistances are chosen so the resistancesmeasured by the taps correspond to the requirements of each branch ofthe voltage divider.

In use, the various taps of the voltage divider are adjusted so theyproduce a null indication on the meter 40, and the weight is readdirectly on the voltage divider 36. Specifically, the positions of thetaps shown in FIG. 4 of the drawing indicate a total weight of 100,000pounds in Branch A, 30,000 pounds in Branch B, 5,000 pounds in Branch C,and 800 pounds in Branch D. The voltages proportional to theseresistances are summed in amplifier 38 and indicate a total weight of135,800 pounds.

The portion of the computer circuit shown in FIG. 5 and indicated by thereference numeral 43 relates to means for directly indicating the centerof gravity of the airplane in terms of the percent MAC. In this circuit,the voltage output of the pressure transducers 26, 28 and 30corresponding to the weight carried by the landing gears, is fed intosumming amplifier 34, along with a voltage from resistances R and Rcorresponding to the unsprung weight of the airplane. The output fromthe summing amplifier 34 at point W in the circuit 43 is a voltageproportional to the total weight of the airplane W Referring to Equation8, it is seen that one of the factors which must be determined is L lMAC L) 9 If the voltage proportional to W serves as an input toamplifier 42, then the output voltage of amplifier 42 would where allthe factors on the right hand side of Equation 10 are known and fixedcharacteristics of a particular airplane, then the voltage at point X inthe circuit will be proportional to 1 MAC Z) (s) The transducer in thenose strut sends out a voltage proportional to the weight carried by thestrut. This voltage serves as one input to the summing amplifier 44. Theother input to summing amplifier 44 comes from the series connectedresistances R and R which produce a voltage proportional to the unsprungweight of the nose landing gear. With this arrangement the sum of theinput voltages to amplifier 44 is proportional to W the total weight onthe nose of the airplane. Consequently, the output of the summingamplifier 44 is a voltage proportional to the total weight on the noseof the airplane W Since we are dealing with a summing amplifier theoutput of summing amplifier 44 is also equal to to e ual L MAC 11 It isnoted that the resistances R and R in summing amplifier 44 are the sameas in summing amplifier 34 in FIG. 1 in order to minimize the number ofdifferent parts in the computing circuit shown in FIG. 7. This, ofcourse, fixes the magnitude of resistance R because as stated above R10must equal Consequently, the voltage at the output of amplifier 44 atpoint y of the circuit will be proportional to MAC (Wm) 12 The above isthe second factor required in the solution of Equation 8.

To this point it can be seen that the percent MAC circuit shown in FIG.5 produces two voltages, each proportional to the factors in Equation 8described above.

The final factor to be determined, but which involves an unknown is thevoltage proportional to MAC Wt It Will be recalled that the sum of allthree factors in Equation 8 equals 0 and voltages proportional to two ofthe factors (9) and (12) present in Equation 8 are known. Furthermore, avoltage proportional to W is at terminal W in circuit 43. Consequently,if this voltage is multiplied by an adjustable calibrated voltagedivider 46 and this multiplied voltage is summed up with voltagesrepresenting the factors (9) and (12) in amplifier 38; then by adjustingthe voltage divider until the null meter 40 indicates 0, the calibrationon the voltage divider would equal y/MAC, which as stated above is thepercent MAC.

The percent MAC voltage divider shown in FIG. 6 operates in thefollowing way. Practical consideration requires the percent MAC to bebetween the limits l +l which are 10 percent and 30 percent of thepercent MAC as measured from the leading edge of the MAC for allconventionally designed airplanes. As seen, the voltage divider 46comprises three branches, J, K, and L. The total resistances in eachbranch in the particular embodiment shown is 15K. Referring to Branch 1,since as stated above, the percent MAC for all practical purposes mustlie between 10 percent and 30 percent of the MAC, then the three tapsshown should correspond to 10 percent each. It is evident that attermnal 3 we want 30 percent of the voltage at G(W and at terminal 2 Wewant percent and at terminal 1 we want 10 percent. Consequently, it canbe shown that R must be 70 percent of the total branch resistance and Rand R and R would be 10 percent each of the branch resistances.

Branch K has 9 taps ranging from 1 percent to 9 percent MAC while BranchL has 9 taps and ranges from .1 percent to .9 percent. The magnitudes ofthe resistances R and R as well as the remaining resistances in thevarious branches are calculated in the same way as the resistances inBranch I. As shown in FIG. 6, the voltage divider indicates a percentMAC of 24.8.

In use, the airplane shown in FIG. 1 is loaded. Then the sliders on thevoltage divider 46 are adjusted until meter 40 indicates a null reading.Then the percent MAC is read directly on the voltage divider. If no nullreading is obtainable by this instrument, it is an indication that theairplane is loaded incorrectly.

The complete circuit 48 shown in FIG. 7 is characterized by a 9 stack 6position switch 50, see FIG. 8. As seen in FIG. 9 switch 50 comprises acentral shaft 52. Nine rotary sliders 54a, 54b, 54c, 54d, 54e, 541, 54g,54k and 54 are attached to shaft 52 and rotate therewith. Each shaftrotates against an identical contact board 56. On each contact board aresix contacts disposed on the arc of a circle and in position to becontacted by the above-described sliders. Each of the sliders has thesame 6 angular position with respect to shaft 52 and they each engage acorresponding contact at the same time.

In particular, as shown in FIG. 8, each of the contact boards 56a, 56b,56c, 56d, 566, 561, 56g, 56/2 and 561' has 6 contact positions inangularly spaced relationship to each other. These contact positions, asshown in FIG. 8 are the weight position, the percent MAC position, thebalance position, the weight calibration position, the weight checkposition and the percent MAC check position. As shown in FIG. 8, allnine sliders engage the weight contact position. In this position, theweight contact circuit 32 shown in FIG. 3 is produced. When the slidersare rotated to the contact identified as percent MAC, the circuit 43shown in FIG. 5 is produced.

The contact identified as the balance position shown in FIG. 8 is foradjusting the summing amplifiers 34, 38, 42 and 44, so they have a zerovoltage output when there is a zero voltage input. If this were not so,error would be introduced into the circuit. Consequently, a study ofcircuit 48 shows when the sliders are in the balance position there is azero input at each of the above-described amplifiers. If this is done,then a voltage meter is applied to the output of each summing amplifier.Next the amplifiers may be adjusted internally by any suitable means toproduce a zero output. It is noted that this internal adjustment of theamplifiers is done very infrequently and occurs usually during anoverhaul.

The contact identified as weight calibration is for the purpose ofadjusting slider 58 of the variable resistance R This slider is in thefeed back circuit of the amplifier 34 and represents the variable gainof a meter of amplifier 34. Initially this adjustment is made byweighing the entire airplane by some independent means. Since theupsprung weight of the airplane is known and is a constant, the weightvoltage divider is adjusted to reflect the total weight of the airplaneminus the unsprung weight.

In the weight calibration position, the voltage representing theunsprung weight is disconnected from the circuit. Then the variableresistance R is adjusted to get a null reading at the meter 40 in orderto adjust the gain and hence the proportionality factor of amplifier 34.The adjustment of resistance R is also done very infrequently and thenonly when the instrument is being recalibrated or repaired. Afterresistance R is adjusted, the total weight of the airplane, includingthe unsprung weight, is adjusted into the voltage divider 36. The sliderof the potentiometer R is then adjusted to get a null reading and thisintroduces a correct factor for the unsprung weight.

To calibrate the percent MAC circuit shown in FIG. 5, the slider 66 ofthe variable resistance R must first be adjusted. This is done by takinginto consideration the fact that the voltage on slider 66 must beproportional to the nose unsprung weight. In the present circuit thisadjustment is determined independently because the nose unsprung weightis a known constant for the particular airplane. Consequently, slider 66may be adjusted by means of a volt meter connected across the resistanceR in a manner well known in the art.

After slider 66 is adjusted, the position of the sliding contact 54 isrotated to the percent MAC position of the six-position 9-stack switch.Then the percent MAC voltage divider is set to reflect the actualpercent MAC. This value is also known at the time of calibration becauseas stated above the weight on the nose landing gear W and the totalweight of the airplane W, are measured independently when the weightcircuit is calibrated. From these factors, and using Equation 8 or usingother means the percent MAC is known. This value is then adjusted intothe percent MAC voltage divider. Then the resistance R of the summingamplifier 44 is adjusted so that meter 40 gives a null reading.

It is desirable to periodically check the circuit 48 to see if it isoperating properly. To do this, transducers 26, 28 and 30 are eachprovided with a center tap 60, 62 and 64, see FIG. 7. At the weightcheck position each of these transducers will produce a voltage and thevoltage divider 36 is adjusted so null meter 40 indicates a zero readingand the calibrations of the setting of the voltage meter under thesecircumstances is recorded. These voltages and the setting of the voltagedivider 36 required to get a null reading on meter 40 are independent ofthe loading configuration of the airplane. With this arrangement thecircuit can be tested to determine whether it is operating correctly bysimply adjusting the slider 54 to the weight check position and thenadjusting the voltage divider to the predetermined calibrated setting.If the meter 40 then indicates a null reading, the weight circuit isoperating correctly. Similarly, when the sliders 54 are rotated to thepercent MAC check position, the voltages at the center taps 60, 62 and64 are fed into the percent MAC circuit 43 in FIG. 5. Then the percentMAC voltage divider 46 is adjusted to a zero position and the calibratedpositions of the voltage divider are noted. This reading also isindependent of the loading configuration of the airplane. Then later,when it is desired to check the percent MAC circuit, the sliders 54 arerotated to the percent MAC check position and the percent MAC voltagedivider is adjusted to the predetermined above-noted setting. If a nullreading is obtained or the deviations from a null reading are withinpermis sible limits this provides an indication that the circuit isoperating correctly.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventioncan be practiced otherwise than as specifically described.

We claim:

1. A device for providing a direct indication of the percent MAC of anairplane having port and starboard landing gears and a third landinggear in longitudinally spaced relation to a line connecting said portand starboard landing gears, comprising a computing circuit for solvingthe equation yI'Vt t. L MAC MAC L MAC' where:

L is the distance between the nose landing gear and the port andstarboard landing gears,

is the percent MAC,

said computing circuit having three parts, the first part comprising anadjustable voltage multiplying means calibrated in terms of percent MACfor multiplying an input voltage by 1/ XIOO the second part comprises asecond voltage multiplying means for multiplying an input voltage by theratio L 11 M AC L the third part comprises a third voltage multiplyingmeans for multiplying an input voltage by the ratio L/MAC, the inputvoltage to said first part comprising a voltage proportional to Wwhereby the output voltage of the first part is a voltage proportionalto 1! MAC X W means in said second part for providing a voltage phasereversal and the input voltage to said second part comprising a voltageproportional to W whereby the output voltage of said second part is avoltage proportional to L 1-1 MAC L the input voltage to said third partcomprising a voltage proportional to W whereby the output voltage ofsaid third part is a voltage proportional to L MAC said computingcircuit having a summing device, the output voltages of said three partsconnected to the input of said summing device, the output of saidsumming device connected to a null meter whereby when the sum of saidoutput voltages of said three parts is zero the calibrations on theadjustable voltage multiplying means indicates the percent MAC.

2. The device described in claim 1 wherein said computing circuitincludes transducers built into the port and starboard landing gears andthe third landing gear, a first summing device, the output of saidtransducers fed into said first summing device to obtain a voltageproportional to W the output of the transducer built into the thirdlanding gear providing a voltage proportional to W 3. The devicedescribed in claim 2 wherein the output of said first summing device isconnected both to the input to said adjustable voltage multiplying meansand to said second voltage multiplying means, the output of thetransducer connected to the third landing gear connected to the input ofthe third voltage multiplying means.

4. The device described in claim 3 wherein said adjustable voltagemultiplying means comprises a three branch voltage divider calibrated interms of percent MAC, and said second and third voltage multiplyingmeans are both summing amplifiers.

5. A device of the class described for use with an airplane having portand starboard landing gears and a third landing gear in longitudinallyspaced relation to a line connecting said port and starboard landinggears comprising separate load transducers built into each of saidlanding gears for converting the load carried by said landing gears intoseparate electrical potentials, a first summing amplifier, the output ofeach transducer connected to the input of said first summing device toobtain a voltage proportional to the total weight of the airplane, anadjustable voltage divider calibrated in terms of the weight of theairplane for producing an adjustable voltage 180 out of phase with thevoltage proportional to the weight of the airplane, a second summingamplifier, switch means having a plurality of positions, in one positionof the switch the voltage proportional to the total weight of theairplane and the adjustable voltage output from the voltage dividerconnected to the input of the second summing amplifier, a null meterconnected to the output of the second summing amplifier, whereby whenthe first voltage divider is adjusted so the null meter indicates theoutput of the second summing amplifier is zero, the calibrations on thefirst adjustable voltage divider indicates the total weight of theairplane.

6. The device described in claim 5 including a second adjustable voltagedivider calibrated in terms of percent MAC, in one of the positions ofsaid switch means the voltage proportional to the weight of the airplanecomprising an input to the said second adjustable voltage divider tomultiply the voltage proportional to the total weight of the airplane bya factor determined by the setting on said secod adjustable voltagedivider, the output of said second adjustable voltage divider comprisingan input to said second summing amplifier, said device producingadditional voltages proportional to the Weight carried by the thirdlanding gear, said additional voltages serving as additional inputs tosaid second summing amplifier, said additional voltages related to thecalibrations of said second adjustable voltage divider in such a Waythat when said second adjustable voltage divider is adjusted so the nullmeter indicates a zero output, the calibration on the second adjustablevoltage divider directly indicated the percent MAC.

7. The device described in claim including a computing circuit forsolving the equation L is the distance between the nose landing gear andthe port and starboard landing gears,

W is the total weight ofthe airplane,

W is the weight carried by the third landing gear,

I is the distance of the nose landing gear from the leading edge of theMean Aerodynamic Chord,

y is the distance between the leading edge of the Mean Aerodynamic Chordand the center of gravity of the airplane,

MAC is the length of the Mean Aerodynamic Chord, and

Z! M AC is the percent MAC,

said computing circuit having three parts, the first part comprising anadjustable voltage divider calibrated in terms of percent MAC formultiplying an input voltage y l/ M A C X 100 said voltage proportionalto the weight of the airplane connected to the input of said secondvoltage divider, the second part comprising a third summing amplifierfor multiplying an input voltage by the ratio L Z MAC L) the inputvoltage to said second part comprising a voltage proportional to thetotal weight of the airplane, the output voltage of said second part 180out of phase with said first part, said third part comprising a fourthsumming amplifier for multiplying an input voltage by the ratio L/MAC,the input voltage to said third part comprising a voltage proportionalto the total weight carried by the third landing gear, the outputvoltage of all three parts connected to the input of said second summinganiplifier in another position of the switch whereby when the secondadjustable voltage divider is adjusted to provide a zero output on thenull meter the calibrations on the second summing amplifier provide adirect indication of the percent MAC.

8. The device described in claim 7 wherein said switch has a thirdposition, said computing circuit connected to said switch means in sucha Way that when said switch means is in said third position the input toeach summing amplifier is zero whereby each summing amplifier can beadjusted to have a zero output when there is a zero input.

9. The device described in claim 8 wherein each of the transducers builtinto the landing gears has a center tap, said switch having a fourthposition, in said fourth position the center tap voltages of thetransducers connected to the input of said first summing device wherebyif said adjustable voltage divider calibrated in terms of percent MAC isadjusted to provide a zero reading on the null meter, the calibrationson the voltage divider required to provide this zero reading will beindependent of the loading configuration of the airplane, therebyproviding a means for determining the performance of the computingcircuit.

10. The device described in claim 5 wherein each of the transducersbuilt into the landing gears have a center tap, said switch havinganother position, in said other position the center tap voltages of thetransducers are connected to the input of the first summing amplifierwhereby if said adjustable voltage divider calibrated in terms of weightis adjusted to provide a zero reading on the null meter, thecalibrations on the voltage divider required to provide this zeroreading will be independent of the loading configuration of the airplane thereby providing a means for determining the performance of thedevice.

References Cited UNITED STATES PATENTS Re. 23,945 2/ 1955 Kolisch235150.2 2,540,807 2/1951 Berry 7365 2,559,718 7/1951 Goodlett et al.235-1502 X 2,686,426 8/1954 Kolisch 73-65 2,725,193 11/1955 Kolisch235150.2 3,063,638 11/1962 Kolisch 235150.2

EUGENE G. BOTZ, Primary Examiner R. W. WEIG, Assistant Examiner US. Cl.X.R. 7365

